Liquid crystals are used primarily as dielectrics in indicating devices, since the optical properties of such substances can be influenced by an applied voltage. Electro-optical devices based on liquid crystals are well-known to the person skilled in the art and can be based on various effects. Examples of such devices are cells having dynamic scattering, DAP cells (deformation of aligned phases), guest/host cells, TN cells ("twisted nematic") and STN cells ("super-twisted nematic") having a twisted nematic structure, SBE cells ("super-birefringence effect"), phase change cells having a cholesteric-nematic phase transition and OMI cells ("optical mode-interference"). The most common indicating devices are based on the Schadt-Helfrich effect and have a twisted nematic structure.
Further, electro-optical devices based on chiral tilted smectic liquid crystals are proposed in Appl. Phys. Lett. 36, 899 (1980) and in Recent Developments in Condensed Matter Physics 4, 309 (1981). In this case, the ferroelectric properties of these materials are made use of. As tilted smectic phases there are suitable, for example, smectic C, F. G, H, I and K phases. There are generally preferred smectic C phases, which permit especially large response speeds. The chiral tilted phases are usually denoted as S.sub.C.sup.*, S.sub.F.sup.* etc., with the asterisk indicating the chirality.
The liquid crystal materials must have a good chemical and thermal stability and a high stability towards electric fields and electromagnetic radiation. Further, the liquid crystal materials should have a low viscosity and in the cells should give short response times, low threshold potentials and a high contrast. Furthermore, at usual operating temperatures they should have a suitable mesophase, for example a nematic, cholesteric or chiral tilted smectic phase. Other properties such as the electrical conductivity, the dielectric anisotropy and the optical anisotropy must fulfill different requirements depending on the type of cell and field of use. For example, materials for cells having a twisted nematic structure should have a positive dielectric anisotropy and an electrical conductivity which is as small as possible. In addition to the general interest in liquid crystal materials having a high optical anisotropy there has recently been an increased interest in materials having a low optical anisotropy, especially for actively addressed liquid crystal devices, for example, in TFT applications (thin film transistor) in television sets. On the other hand, chiral tilted smectic liquid crystals should have a sufficiently high spontaneous polarization.
In order to optimize the properties, liquid crystals are generally used as mixtures of several components. It is therefore important that the components have a good miscibility with one another. Cholesteric mixtures preferably include one or more optically active doping substances and a nematic liquid crystal material, and ferroelectric liquid crystals preferably include one or more optically active doping substances and a liquid crystal material having a tilted smectic phase.